Electrical furnace for melting thermoplastic material

ABSTRACT

A furnace for electrically heating thermoplastic material preferably including a chamber for containing a body of thermoplastic material defined by an upstanding peripheral wall and a bottom wall symmetric about a vertical center-line through the chamber; a first set of six electrode groups extending through the peripheral wall in generally the same horizontal plane, with each group having geometric center and with each group center being equidistant from adjacent group centers; a second set of six electrodes extending through the peripheral wall at generally the same height at those in the first electrode set, with each second set electrode being positioned equidistantly between adjacent first set group centers; and a supply circuit means for providing intraphase cross-firing current paths, through said thermoplastic material, between diametrically opposite first set electrode groups, intraphase peripheral-firing current paths between once-removed electrodes within the second electrode set, and interphase current paths between all electrodes not connected by intraphase current paths. The supply circuit means may include two sets of transformers having three transformers each and each set may have its transformers delta-connected to mainlines of a three-phase power source. The secondaries of one set of transformers is preferably double-delta connected to the six electrodes of the second electrode set, with the deltas thereof being reverse-phase rotated. The other transformer set has its secondaries connected across diametrically opposite pairs of first set electrode groups, with the electrodes in each group being connected in parallel to the respective transformer secondaries. The transformer secondaries may be provided with adjustable taps and two motors may be connected to the taps of respective transformer sets, whereby the taps of one transformer set may be multiply adjusted independently from the taps of the other transformer set.

United States Patent Rutledge et 'al.

[ 5-l Dec. 3, 1974 ELECTRICAL FURNACE FOR MELTING THERMOPLASTIC MATERIAL [75] Inventors: Heath A..Rutledge; Myron A. Smith;

Paul F. Spremulli, all of Corning,

[73] Assignee: Corning Glass Works, Corning,

221 Filed: Mar. 28, 1974 [21] Appl. No.: 455,853

[52] US. Cl. 13/6 51 Int. Cl (3031) 5/02 [58] Field of Search 13/6, 20, 22

[56] References Cited UNITED STATES PATENTS 3,520,979 7/1970 Scarfe et al 13/6 X 3,725,558 3,742,lll

4/1973 tSerstevens....

6/1973 Pieper Primary ExaminerR; N. Envall, Jr; v 1 Attorney, Agent, or Firr'n-Thomas J. McNaughton; Clarence R. Patty, Jr.

571 ABSTRACT A furnace for electrically heating thermoplastic material preferably including a chamber for containing a body of thermoplastic material defined by an upstanding peripheral wall and a bottom wall symmetric about a vertical center-line through the chamber; a first set of six electrode groups extending through the peripheral wall in generally the same horizontal plane, with each group having geometric center and with each group center being equidistant from adjacent group centers; a second set of six electrodes extending through the peripheral wall at generally the same height at those in the first electrode set, with each second set electrode being positioned equidistantly between adjacent first set group centers; and a supply circuit means for providing intraphase cross-firing current paths, 'through said thermoplastic material, be-

tween diametrically opposite first set electrode groups,

intraphase peripheral-firing current paths between once-removed electrodes within the second electrode set, and interphase current paths between all electrodes not connected by intraphase current paths. The supply circuit means may include two sets of transformers having three transformers each and each set may have its transformers delta-connected to mainlines of a three-phase power source. The secondaries of one set of transformers is preferably double-delta connected to the six electrodes of the second electrode set, with the deltas thereof being reverse-phase rotated. The other transformer set has its secondaries connected across diametrically opposite pairs of first set electrode groups, with the electrodes in each group being connected in parallel to the respective transformer secondaries. The transformer secondaries may be provided with adjustable taps and two motors may be connected to the taps of respective transformer sets, whereby the taps of one transformer set may be multiply adjusted independently from the taps of the other transformer set.

8 Claims, 6 Drawing Figures PATENTEL 3|974 3,852,508

' SHEEI 20$ 3 PATENTEL BEE 3 74 sum 3 0F 5 BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention relates to apparatuses for heating thermoplastic material and more particularly to furnaces of the kind comprising a furnace chamber having a bottom wall and an upstanding peripheral wall for containing the thermoplastic material in its unmelted and molten states, electrodes spaced apart in plan for passing current through the thermoplastic material, and an electrical supply circuit for supply alternating electric current to-the electrodes.

2. Prior Art Electrically heated furnaces .of the type described in U.S. Pat. No. 3,524,206, have recently been further developed for the purpose of melting relatively large volumes of glass-making materials. With such development, the diameter of the furnace chamber has. been substantially increased; and in order to provide the additional electrical energy neededto melt the increased volume of thermoplastic material, more electrodes have been employed. Particularly, in lieu of. the flushtype electrodesshown in the aforementioned patent,a relatively large plurality of rod-type electrodes have been used. 4

.One' of the major problems encountered in melting glass utilizing such a tankof large diameter and with a In the prior art furnaces, such as those disclosed in U.S. Pat. Nos.- 3,585,861, 3,524,206, 3,440,321 and 2,993,079, electrodes positioned generally at the same height within the chamber were used to provide,

through the thermoplastic material, either intraphase I cross-firing current paths or intraphase peripheralbecause the rate of melting, andhence the rate at which glass is-produced by the furnace, is determined by the area of contact-between the batch material 22 and the heated molten material 24. i Another problem relates. to contamination of the refining zone of the furnace with unmelted or partially melted batch material. The refining or quiescent zone lies in the vicinity of the bottom wall of the chamber. The refining zone results from the fact that the temperature of the molten glass in the bottom part of the tank is substantially uniform horizontally, while a vertical temperature'gradient exists in this part of the tank, the

coolest glass being in the vicinity of the bottom wall of the tank. Contamination of this zone occurs when the fusion line is allowed to extend too deeply toward the bottom wall and also when the batch material is allowed to be drawn downwardly into this zone by convection currents along the peripheral walls of the tank between adjacent electrodes. Accordingly, it is necessary, at each rate of pull, to not only maintain the temperatures throughout theperipheral areas sufficiently high to prevent downward convection of unmelted batch material or unfined glass in this area, but it is also necessary to maintain the temperatures at the center of the melting zone of the chamber sufficiently high to firing current paths, but not both intra-phase crossfiring and intraphase peripheral-firing current paths. A shortcoming of those furnaces whichhave means for providing only intraphase cross-firing current paths was that the downward convective flow between electrodes could 'only be prevented by increasing the temperatures in the vicinity of the electrodes; however, such increase substantially reduces electrode and wall life and also reduces the area of contact between the batch and molten material and thus causing a reduction in the melting rate of the tank.

In those furnaces having means for providing only intraphase peripheral-firing current paths between electrodes generally at the same chamber height, in order to provide sufficient heat in the center of the tank and thus to prevent the batch from descending therethrough into the refining zone, it was also necessary to increase the temperatures at theelectrodes; and, in addition to theincreased corrosion of the electrodes and adjacent wall portions, such increase in. peripheral temperatures tends to melt away a peripheral portion of the batch cover,-thus allowing an undue amount of thermal energy to escape from the bath.

The main object of the present invention is to provide a new and improved apparatus for melting thermoplastic material better adapted to overcome these .problems than these hitherto available.

SUMMARY OF THE INVENTION The apparatus for electrically heating thermoplastic material of the present invention comprisesa chamber for containing a body of thermoplastic material defined by. a bottom wall and an upstanding peripheral wall symmetricabout a vertical center-line through the geometric center of said bottom wall; fi-rstand second. sets of six electrode means positioned'generally symmetrically about said chamber center-line, for providing, re-- for providing equal intraphase current'flow between diametrically opposite pairs of first set electrode means, and second conductor means connecting the other set of voltage sources to said'second set of electrode means for providing equal intraphase current flow between pairs once-removed second set electrode means and for providing uniform interphase current flow between all remaining pairs of electrodemeans.

The electrode means of said first and second sets of electrode means may be comprised of one or more rodtype electrodes. The six groups of first set electrode means are positioned equidistantly from each other and symmetrically about the center-line of the furnace chambenEach of second set electrode means are positioned equidistantly between said first set electrode means and also symmetrically about said chamber center-line.

The sets of voltage sources preferably include three transformers per set, with the primaries of each set of three transformers being delta-connected across the three main'lines of a three phase source of electrical power. The three secondaries of one set are preferably connected across three pairs diametrically opposite first electrode means. The six secondaries of the other set of transformers are preferably double-delta connected, with reverse phase rotation between the deltas,

to said six second set electrode means, with the first delta connected to three non-adjacent second set electrode means and with the second delta connected to the remaining second set electrode means.

The supply'circuit means may further include regulator means for independently controlling the electrical energysupplied to respective first and second sets of electrode means. Such regulator means may include adjustable step transformers; that is, the transformers within said two sets of transformers mayeach have adjustably tapped secondaries, wherebythe secondaries of one set of transformers may be multiply adjusted intherein and the supply circuit for feeding current to the electrodes.

The furnace chamber may be generally circular in form having a bottom wall 16 and an upstanding pebottom wall of the furnace. Batch 22 forms a fusion dependently from'thesecondaries of the other set of transformers, and vice versa.

BRlEF DESCRIPTION'OF THE DRAWINGS FIG. 1 is a vertical sectional'view ofa furn acein ac-,

cordancewith a preferred embodiment of the invention. i

FIG. 2 is across-sectional diagrammatic view of the furnace of FIG." I, with, however, the thermoplastic material being removed for the sake of clarity, and also includes a schematic wiring diagram for an arrangement of electrodes employed therein.

FIG. 3 is a partialvertical sectional view of one embodiment of the present invention and illustrates an electrode arrangementwherein firstand second sets of electrode means he in twoseparate planes'within the same'heatingzone; u

;'FIG. 4 is a partial vertical sectional view of another embodiment of the" furnace of the present invention and illustratesan electrode arrangement wherein the cross-firing first set of electrode means are positioned generally at the same height as that of the peripheralfiring second set of electrode means.

- FIG. 5 is a voltage vector diagram representing the voltage relationships between theelectrode means of r the furnace, with the magnitudes of intraphase crossfiring and peripheral-firing voltages being equal.

FIG. 6 is a vector diagram depicting the voltage relationshipsbetween the electrode means of the furnace,

with, however, the magnitude of the intraphase crossfiring'voltages being greater than the magnitude of the intraphaseperipheral-firing voltages.

described and illustrated in US. Pat. No. 3,524,206,

except for'certain modifications in the form and arrangements and numbers of electrodes employed zone or surface depicted by line 23 of generally a conical shape with molten material 24. i i

Disposed in the chamber 14 within a given heating zone are a plurality of electrodes means designated 1 to 12. The electrode means may each be comprised of one or more flush-type or rod-type electrodes, and with regard to rod-type electrodes, their sizedepth of immersion and arrangement will vary according to size and shape of the chamber and type of thermoplastic material to be melted. The electrode meansl-to 6 constitute a first cross-firing set of electrode means, with each electrode means. therein preferably including three rod-type electrodes designated a, band c respectively. The electrode means 7 to 12 constitute a second peripheral-firing set of electrode means, with each electrode means'therein preferablyincludingone rodtype electrode; however, each electrodemeans7l2 may include more than one electrode. As shown in FIGS. 3 and 4 each of the electrodes a, b and c of the first set of electrodemeans 1 through '6 may be disposed within'the same heating zone above bottom wall 16. As illustrated in FIG. 3 each of the electrodes d within the second set of electrode means 7 through 12 may be positioned .at a height belowthe plane of electrodes within saidfirst set; or, as shown in'FIG.,4 the electrodes of electrode means 7 to 12 may be positioned at the same height above bottom wall 16 as that .of electrodes at, b and c of electrodemeansl to 6. r .As shown in FlG 2 electrode bwithin electrode means 1 to 6 is positioned at the elevational geometri-- cal center C of said means. As shown in FIG. 2', the first set of electrode means 1 to 6 are positioned about the periphery of chamber 14 such that the geometrical centers C or electrodes b, of each electrode means 1 to 6 are equidistant from the centers of the two adjacent first set means-centers. I

Since the preferred embodiments of the-invention includes only one electrode din each electrode means of the second set of electrode means 7 through 12, the elevational geometric center C ofthe'second means are located at the geometric centers of electrodes. Each second setelectrode meansis peripherally positioned Current is fed to thefirst and second, set of electrode means from a polyphase electrical supply system which may be the main supply available inthe area in which the furnace isin operation. In this case such system would normally be a three phase supply system. ln FIG.

2 main lines of three-power phases of a standard main line supply are indicated by reference numerals L,, L and L The current is fed from this system to the electrode means through the intermediary supply circuit adapted to provide two independently controllable sets of voltages sources, such that voltage supplied to the electrode means of the first cross-firing set of electrode means 1 to 6 may be controlled independently from that supplied to the second peripheral-firing set of electrode means 7 to 12. More specifically, the supply circuit means includes a first set of voltage sources adapted to provide intraphase cross-firing current paths, shown at least approximately as dashed lines in FIG. 2, between diametrically opposite electrode .means of said first electrode means set, that is, between electrode means 5 and 2, land 4, and 3 and 6. The supply circuit means further includes a second set of voltage sources adapted to provide intraphase peripheralfiring paths between once-removed electrode means within respective second electrode means sets 7-12, that is, as shown at least approximately by solid lines in FIG. 2, between electrode means 7 and 9, 7 and 11, 8

and 10, 8 and 12, 9 and 11, and 12 and 10. The supply circuit means also preferably includesmeans for varytraphase and interphase voltages between all electrode means remains symmetrical'about the center-line 17 of chamber 14.

The first set of voltages'sourcespreferably includes a first settransformer Tl having three primary windings 21, 23 and 25 which are delta connected across main lines L1, L2 and L3. The secondary, windings of said first set transformer T1, indicated at reference numerals 22, 24 and 26, are connected to} diametrically opposite electrode means within said first electrode means set; that is, secondary winding 22 is connected to electrode means 1 and 4, winding 24 is connected to electrode means 3 and 6, and winding 26 is connected across electrode means 5 and 2. Within each electrode means, electrodes a, b and 0 thereof are connected in parallel to the respective terminals of said secondary windings.

.The second set of voltage sources may include a second set transformers T2 having three primary windings 27, 30 and 33 which are delta connected across main lines L1, L2 and L3, respectively. The secondary windings of said second set of transformers T2, indicated at reference numerals 28, 29, 31, 32, 34 and 35, are double delta connected to said second set electrode means, with reverse phase rotation between each of the two deltas so that secondary windings 29, 32 and 35 comprise one delta connection and secondary windings 28, 3! and 34 comprise the other delta connection. its phase rotation being reversed from said first-mentioned deltaconnection. More specifically, secondarywindings 29,32 and 35 are delta connected respectively across once-removed second set electrode means 10 and 8, l2 and 10, and 8 and 12. Similarly, but with reverse phase rotation, secondary windings 28, 31 and 34 are delta connected respectively across electrode means 7 and 9, 9 and 11, and 11 and 7.

The secondary windings of transformers T1 and T2 may be provided with adjustable-step taps. Two servomotors, each connected respectively to the secondary winding taps of the transformers T1 and T2, may be usedto multiply adjust the taps of said'first set of voltage sources independently from those of said second set of voltage sources, and vice versa. Alternatively buck-boost regulator means, suchas saturable reactors or variable auto-transformers may be connected between transformers T1 and T2 and main lines L1, L2 and L3 in order to independently control the polyphase electrical power supplied to transformersTl and T2. However, in order to ensure that the phase voltages applied across each of the electrode means of said first and-second electrode means sets is uniform and symmetrical about center-line 17, the turns ratio of each of the primary and secondary windings within each transformer T1 and T2 sould be selected to provide voltages of equal magnitude across the secondary windings of transformer T1 and, similarly, but not necessarily same voltages as those of transformer set T1, voltages equal magnitude across the secondaries of transformer set T2. v

The magnitudes of cross-firing voltages furnished by secondary windings 22, 24 and 26 are equal to each other, as represented by the phase voltage vectors or phasors V V and V in FIGS. 5 and 6, with vectors V V and V representing the respective voltages between electrode means 5 and 2, l and 4, and 3 and 6. The voltages furnished by secondaries 29, 32 and 35 of transformer T2 are also equal to each other, as is represented by voltage vectors V5, V and V,,, respectively. The voltages furnished by secondaries 28, 31 and 34 are equal to each other and to the voltages provided by secondary windings 29, 32 and 35, as is represented by vectors V V and V Thevoltage vector diagram of FIGS; 5 and 6 illustrate not only thesymmetry of the intraphase crossfiring, represented by vectors V V and V and of the intraphase peripheral-firing, represented by vectors V, through V,; but they further depict the symmetry of the .interphase voltages about the center-point 0 between electrode means of the first set shown in part by V, and V,,; between electrode means of the second set shown in part by V, and V,; between electrode means of the first set and electrode means of the second set shown in part by V,,, V,., V,.. As shown by FIG. 5, wherein the magnitude of vectors V V and V, each has been in creased, both the intraphase and interphase voltage vectorsremain symmetrical about center point 0, even though the magnitude of potential across the crossfiring paths between the first set of electrode means has been increased independently from the magnitude of potential applied across the peripheral-firing paths of second set of electrode means.

cross-firing vectors by 30 FIGSQS' and 6 illustrate the relationships of the interphase voltages between electrode means, The magnitude of interphase'voltages V through V, can be determined according to well-known formulas defining the lengths of sides of triangles. For example, the magnitude of voltages, V; between any two adjacent electrodes can be computed according to the following formula:

where V equals the magnitude of vectors V V and V and V,,, equals the magnitude of vectors V through V Similarly, the magnitude of interphase vectors V through V, may be determined as a function of magnitudes of vectors, V and V that is, as a function of the.

magnitudes or potential across the secondary windings of said sets of transformers T1 and T2, respectively.

More specifically, in order to obtain a symmetrical distribution of intraphase cross-firing and peripheral firing current paths'and interphase current paths between the twelve electrode means about center-line 17,

as shown in FIG. 2, itis not necessary to adhere to strict in FIG. 5, with the resultingvector diagram, as shown in FIG. 6, depicting a polygonal figure within which vectors V, would be drawn being different from the polygonal figure constructed'by joining the aforementioned electrodes. v

It should be noted that the first and second sets of electrode means may lie in two planes or may lie at same general height above the chamber bottom wall. It is only essential that there be interphase firing between all respective electrode means within each of the first and second; sets, that is,'between electrode means not connected by said interphase cross-firing and peripheral-'firingpaths within eachrespective set; It will be appreciated that such interphase firing paths, in conjunction with the intraphase firingipaths. constituting a heating zone. -That is. a heating zone is that area within the bath wherein the sets of electrode means are not electrically isolated from oneanother.

More than one heating zone may be utilized to control the heating and fining of the thermoplastic material. As shown in FIG. 1, three zones may be used, with the uppermost and middle zone being utilized to melt and fine the thermoplastic material and the lowermost zone being used during the start-up procedure.

inasmuch as'the foregoing descriptions have been b. a first set of six equidistantly means ,for providing, through said thermoplastic material, at least three cross-firing current paths intersecting in the vicinity of said center-line;

c. asecond set of six electrode means for providing, through said thermoplastic material, intraphase peripherabfiring current paths, being symmetric about said center-line; and I v I d. supply circuit means for transmitting alternating electrical current'to said first and'second set electrode means, including t l. a supply of .polyphase electrical power;

2. first and second sets of voltage sources'fed by said supply, with said first voltage source set furnishing intraphase voltages of respectively equal magnitude and relativephases represented by voltage vectors corresponding at least approximatelyto lines joining said first set electrode means and with said secondvoltage source set equal magnitude and relative phases represented by voltage vectors corresponding at least approximately to lines joining said second set electrode means; and l" i r a 3. conductor means for connecting said first and second sets of voltage sources respectively to said first and second sets of electrode means to apply voltages 'of respectively. equal magnitudes across said at leastthree cross-firing paths, to

across saidfsymmetric peripheral-firing paths,

and voltages of [respectively equal magnitudes between all corresponding pairs of electrode means not connected by said intraphase crossfiring and peripheral firing current paths.

2. Thelfurnace' of claim 1 wherein said supply circuit means includes regulator means for independently controlling the magnitudes of intraphasevoltages supplied by each respective set of voltage sources, whereby the electrical energyprovided along said intraphase, cross: firing and peripheral-firing paths may be separately regulated. 1

3. The furnace of claim 1 wherein each said first set electrode means is positioned equidistantly from adjacent electroder'neans or saidffirst set and each .of said second set electrode means is positioned equidistantly between adjacent first set electrode means. j 4. The furnace of claim 1 whereinthe geometric center of each electrode means within said first and second set of electrode means liefgenerally at the same height above said bottom wall.

. 5. The furnace of claim 1 wherein said polyphase source supplies three phases, and said two sets of voltage sources include respective first and second sets of transformers, and said connector means includes I a. in said first set of transformers, three secondaries being connected across three diametrically oppofurnishing intraphase voltages of respectively apply voltages of respectively equal magnitudes voltage phases of said first-mentioned three electrode means.

6. A furnace for melting and for electrically heating thermoplastic material comprising:

a. a chamber, for containing a body of thermoplastic material, defined by a bottom wall and an upstanding wall symmetrical about a vertical center-line;

b. a first set of six electrode groups extending through said peripheral wall, with each group having a geometric center and with each group center lying in generally the same plane and being equidistant from adjacent group centers;

c. a second set of six electrodes extending through said peripheral wall, with each second set electrode being positioned equidistantly between adjacent first set group centers; and

d. supply circuit means for providing, through said thermoplastic material, intraphase cross-firing current paths between diametrically opposite first set electrode groups, intraphase peripheral-firing current paths between once-removed electrodes within said second set of electrodes, and interphase current paths between all first set groups and sec- ,ond set electrodes not connected by said intraphase cross-firing and peripheral firing current paths.

7. The fumaceof claim 6 wherein said supply circuit means includes:

a. a supply of polyphase power;

b. two sets of voltage sources fed by said supply furnishing voltages of magnitudes and relative phases represented by intraphase and interphase voltage vectors being symmetrical about a center-line corresponding to said vertical center-line; and

c. conductor means for connecting said sets of voltage sources to said first and second sets of electrodes to apply intraphase voltages of a first magnitude across said cross-firing paths between said diametrically opposite first set electrode groups, to apply intraphase voltage of a second magnitude across said peripheral-firing paths between onceremoved electrodes within said second electrode set, and to apply interphase voltage between all first set groups and second set electrodes not connected by said intraphase cross-firing and peripheral firing paths.

8. The furnace of claim 6 further including regulator means for separately controlling the amount of electrical energy provided along said intraphase cross-firing paths and the amount of energy provided along said intraphase peripheral-firing paths.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,s52,509 I e I Dated December 3, 1974 InventOflS) Heath A. Rutledqe, Myron A. Smith & Paul F. Spremulli It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 45, "constituting" should be -constitute--.

Column 8, lines 1 and 2, "a first set of six equidistantly means for providing," should read --a first set of six equidistantly spaced electrode means for providing,--:.

Column 8, line 37, "means includes" should read -means further includes-.

Column l0, line 7, "center-line" should read center point.

Signed and sealed this 4th day of March 1975.

(SEAL) Attest:

. C. PKARSHALL'DANN Commissioner of Patents and Trademarks RUTH C. Z-EASON Arresting Officer ORM PC4050 (10-69) USCOMM-DC 60376-P69 Q U GOVERNMENT PRINTING OFFICE: 1969 0-366-38 UNITED STATES PATENT OFFICE 7 CERTIFICATE OF CORRECTION" Patent No. 3 852,509 Dated December 3, 1974 n Heath A. Rutledqe, Mvron A. Smith & Paul F. Spremulli It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 45, "constituting" should be -constitute--.

Column 8, lines 1 and 2, "a first set of six equidistantly means for providing," should read a first set of six equidistantly spaced electrode means for providing,1-.

Column 8, line 37, "means includes" should read means further includes. Y

Column 10, line 7, "center-line" should read centerpoint.

Signed and sealed this 4th day of March 1975.

(SEAL) Attest:

P my w a C. I'MRSHALL DANN Uh: C .--AQON Commissioner of Patents Attesting Officer and Trademarks .M PO-105O (10-59) USCOMM-DC 6O376-P69 u.s. GOVERNMENT PRINTING OFFICE: I969 0-366-33 

1. A furnace for electrically heating thermoplastic material comprising: a. a bottom wall and an upstanding peripheral wall symmetric about a center-line, with said bottom and peripheral walls defining a chamber for containing a body of thermoplastic material to be melted; b. a first set of six electrode equidistantly-spaced means for providing, through said thermoplastic material, at least three cross-firing current paths intersecting in the vicinity of said center-line; c. a second set of six electrode means for providing, through said thermoplastic material, intraphase peripheral-firing current paths, being symmetric about said center-line; and d. supply circuit means for transmitting alternating electrical current to said first and second set electrode means, including
 1. a supply of polyphase electrical power;
 2. first and second sets of voltage sources fed by said supply, with said first voltage source set furnishing intraphase voltages of respectively equal magnitude and relative phases represented by voltage vectors corresponding at least approximately to lines joining said first set electrode means and with said second voltage source set furnishing intraphase voltages of respectively equal magnitude and relative phases represented by voltage vectors corresponding at least approximately To lines joining said second set electrode means; and
 3. conductor means for connecting said first and second sets of voltage sources respectively to said first and second sets of electrode means to apply voltages of respectively equal magnitudes across said at least three cross-firing paths, to apply voltages of respectively equal magnitudes across said symmetric peripheral-firing paths, and voltages of respectively equal magnitudes between all corresponding pairs of electrode means not connected by said intraphase crossfiring and peripheral-firing current paths.
 2. first and second sets of voltage sources fed by said supply, with said first voltage source set furnishing intraphase voltages of respectively equal magnitude and relative phases represented by voltage vectors corresponding at least approximately to lines joining said first set electrode means and with said second voltage source set furnishing intraphase voltages of respectively equal magnitude and relative phases represented by voltage vectors corresponding at least approximately To lines joining said second set electrode means; and
 2. The furnace of claim 1 wherein said supply circuit means includes regulator means for independently controlling the magnitudes of intraphase voltages supplied by each respective set of voltage sources, whereby the electrical energy provided along said intraphase cross-firing and peripheral-firing paths may be separately regulated.
 3. The furnace of claim 1 wherein each said first set electrode means is positioned equidistantly from adjacent electrode means of said first set and each of said second set electrode means is positioned equidistantly between adjacent first set electrode means.
 3. conductor means for connecting said first and second sets of voltage sources respectively to said first and second sets of electrode means to apply voltages of respectively equal magnitudes across said at least three cross-firing paths, to apply voltages of respectively equal magnitudes across said symmetric peripheral-firing paths, and voltages of respectively equal magnitudes between all corresponding pairs of electrode means not connected by said intraphase cross-firing and peripheral-firing current paths.
 4. The furnace of claim 1 wherein the geometric center of each electrode means within said first and second set of electrode means lie generally at the same height above said bottom wall.
 5. The furnace of claim 1 wherein said polyphase source supplies three phases, and said two sets of voltage sources include respective first and second sets of transformers, and said connector means includes a. in said first set of transformers, three secondaries being connected across three diametrically opposite pairs of electrode means within said first electrode means set, and b. in said second set of transformers, three secondaries being closed delta connected to three once-removed electrode means within said second set of electrode means, and three additional secondaries being closed-delta connected to the other three electrode means with said second set of electrode means, with the voltage phases between the other three electrode means being reversed from the voltage phases of said first-mentioned three electrode means.
 6. A furnace for melting and for electrically heating thermoplastic material comprising: a. a chamber, for containing a body of thermoplastic material, defined by a bottom wall and an upstanding wall symmetrical about a vertical center-line; b. a first set of six electrode groups extending through said peripheral wall, with each group having a geometric center and with each group center lying in generally the same plane and being equidistant from adjacent group centers; c. a second set of six electrodes extending through said peripheral wall, with each second set electrode being positioned equidistantly between adjacent first set group centers; and d. supply circuit means for providing, through said thermoplastic material, intraphase cross-firing current paths between diametrically opposite first set electrode groups, intraphase peripheral-firing current paths between once-removed electrodes within said second set of electrodes, and interphase current paths between all first set groups and second set electrodes not connected by said intraphase cross-firing and peripheral firing current paths.
 7. The furnace of claim 6 wherein said supply circuit means includes: a. a supply of polyphase power; b. two sets of voltage sources fed by said supply furnishing voltages of magnitudes and relative phases represented by intraphase and interphase voltage vectors being symmetrical about a center-line corresponding to said vertical center-line; and c. conductor means for connecting said sets of voltage sources to said first and second sets of electrodes to apply intraphase voltages of a first magnitude across said cross-firing paths between said diametrically opposite first set electrode groups, to apply intraphase voltage of a second magnitude across said peripheral-fIring paths between once-removed electrodes within said second electrode set, and to apply interphase voltage between all first set groups and second set electrodes not connected by said intraphase cross-firing and peripheral firing paths.
 8. The furnace of claim 6 further including regulator means for separately controlling the amount of electrical energy provided along said intraphase cross-firing paths and the amount of energy provided along said intraphase peripheral-firing paths. 